1. Field of the Invention
This invention relates generally to arrangements for welding metals and plastics with multiple focused energy beams, such as laser beams, and more particularly, to a welding arrangement for joining materials of different thicknesses using energy beams having correspondingly different intensities.
2. Description of the Related Art
Several systems are available for welding materials using a beam of energy. With respect to metals, an energy beam, usually a laser, is focused on the metal to be welded to achieve a power density on the order of 1,000,000 watts per square centimeter. The actual power density required will depend upon a variety of factors, including the particular material to be welded, the thickness of the material to be welded, the desired depth of penetration, the rate of travel of the beam along the weld, etc. One technique that is useful for welding metal is generally referred to as "key hole" welding wherein an energy beam having a power density sufficient to form a hole in the material is applied to form a key hole surrounded by molten material. The focused energy beam is then translated with respect to the material to be welded, the molten material flowing around both sides of the key hole and coming together at the trailing edge thereof. Such deep penetration welding vaporizes a thin column of material through the work piece, so as to have a high depth-to-width aspect ratio in the fusion zone. Cooling of the molten material usually happens quickly via conduction through the surrounding unmolten material.
Generally, the end to be achieved by such welding is the attachment of two pieces of material with the weld. One such weld is called the "butt joint," wherein the materials are joined edge-to-edge. However, the materials to be joined do not always meet precisely with one another. Their respective edges may not be straight resulting in gaps, the sheet materials may be warped, or some other nonuniformity or imperfection may be present to prevent a good mating of the surfaces. The resulting gaps in the seam between the materials to be welded affect the amount of energy that is converted to heat in the material and affect the ability of the molten pools of material from each of the sheets to join one another in the trailing region of the key hole. The result is an imperfect weld having weak portions therealong.
Several techniques are established for manufacturing butt joints between sheet materials. The first technique employs a single stationary beam for joining the sheets. This method is well suited for straight lines and material combinations where the difference in thickness is not too great. In regions where the differences in material thicknesses is great, or where the weld to be effected is not linear so as to have tight radii, there is the possibility of dwelling on the thicker or thinner material. If the energy beam is directed during translation excessively to the thick material, the joint quality will be poor due to insufficiency of the quantum of material being melted or liquefied, and poor penetration. This results from the beam being of insufficient intensity. On the other hand, if the beam focuses unduly on the thin material, the intensity of the beam may be too high for the thin material and will result in drop-through of the weld metal.
Alignment of the energy beam is a significant problem, as focusing is generally effected to a tenth of a millimeter. Alignment has to be guaranteed within a tolerance of less than half of the diameter of the laser beam. One known approach to alleviating this problem is to tilt the beam in a plane perpendicular to the direction of weld travel. This method, however, is not sufficiently reliable to accommodate small radii on nonlinear welds, particularly at high weld speeds.
In addition to the foregoing, gaps often are present between the parts to be welded resulting from either improper edge preparation or the complexity of the shape of the components. In addition to the difficulty in filling such a gap with molten material to achieve a high quality seam, it is possible that the beam will propagate through the gap without welding the materials at all. Gapping, therefore, is a significant problem in joining flat sheet materials.
A second approach to manufacturing welded sheet materials is the oscillation of the energy beam in a plane perpendicular to the direction of the weld. Although this arrangement decreases alignment and gap difficulties, it requires a highly sophisticated mirror arrangement that oscillates or rotates at high frequencies and is cooled by water to handle the energy absorption. The ability to cool the mirror limits the power of the energy beam that can be used. In addition, the intensity of the weld spot is not adapted to either of the sheets of material.
More recently, a third manufacturing technique has been developed, which involves splitting the energy beam into two beams having equal intensity. This arrangement, however, provides no advantage when applied to dissimilar material thicknesses.
It is, therefore, an object of this invention to provide a welding arrangement of the type that employs an energy beam, such as a laser, for joining sheet materials having dissimilar thicknesses.
It is another object of this invention to provide a laser welding system that can reduce the deleterious effects of gaps between materials to be welded to one another, and thereby achieve a continuous good weld.
It is also an object of this invention to provide a welding arrangement that can achieve a good continuous weld between materials of dissimilar thicknesses, the weld having a nonlinear configuration.
It is a further object of this invention to provide a welding arrangement having at least two welding beams that have an adjustable distance therebetween.
It is additionally an object of this invention to provide an arrangement for controlling the relative intensities of two energy beams.
It is yet another object of this invention to provide an arrangement for controlling the characteristics of welding beams in response to a thickness characteristic of the material being welded.
It is yet a further object of this invention to provide an arrangement for controlling the characteristics of welding beams in response to a characteristic of a gap between materials being welded.